In the primary recovery of oil from a subterranean, oil-bearing formation or oil and gas reservoir, it is usually possible to recover only a limited proportion of the original oil present in the reservoir. For this reason, a variety of supplemental recovery techniques have been used to improve the displacement of oil from the reservoir rock. These techniques can be generally classified as thermally based recovery methods (such as steam flooding operations), water-flooding methods, and gas-drive based methods that can be operated under either miscible or immiscible conditions. These techniques are commonly categorized under the broad heading of enhanced oil recovery (EOR) techniques.
It may be desirable for gas-based EOR techniques that the EOR fluid and the oil are miscible under the conditions in the formation. In miscible flooding operations, an injection fluid or solvent is injected into the reservoir to form a single-phase solution with the oil in place so that the oil can then be removed as a more highly mobile phase from the reservoir. A fluid miscible with oil may enhance oil recovery by reducing or eliminating the interfacial tension between the oil and water thus improving oil displacement. The injection fluid may also enhance oil recovery, even if it does not form a single-phase solution with the oil, by swelling the oil, reducing viscosity, and/or vaporizing lighter components of the oil. The injection fluid may be a light hydrocarbon such as liquefied petroleum gas (LPG), a hydrocarbon gas containing relatively high concentrations of aliphatic hydrocarbons in the C2 to C6 range, nitrogen, and/or carbon dioxide. The injection fluid typically is a gas at atmospheric conditions, but becomes a dense fluid at reservoir conditions. Often, rail cars and/or truck transports are used to transport injection fluid to a well site for EOR operations, which can lead to increased costs and may prove challenging when access to the reservoir is restricted, e.g., by geography.
Various gas-based EOR flooding techniques known in the art include formulating particular mixtures of gases to increase the effectiveness of gas flooding operations. For example, U.S. Patent Application Publication No. 2015-0060075 discloses an EOR method that utilizes an ether-based injectant to mobilize oil within the hydrocarbon reservoir. Also, U.S. Pat. No. 4,512,400 discloses a method for upgrading natural gas to ethane, propane, and butane constituents using synthesis gas processing combined with the Fischer Tropsch process. The upgraded natural gas is then serially used in a multi-well miscible drive process. However, complicated techniques for formulating and/or synthesizing gas-based injection fluids can be expensive and/or time consuming, and may either require an amount of additional equipment that is infeasible for deployment to certain assets or may require sending products to a distant, centralized facility for processing.
Another supplemental recovery technique that has been used is to re-pressurize the formation. For example, in some circumstances, a portion of gas produced along with oil, so called “associated gas”, may be reinjected back into a gas cap of the hydrocarbon reservoir for pressure maintenance. However, reinjection of associated gas becomes less viable over the lifetime of a hydrocarbon asset as an increasing amount of pressure is needed to maintain production from a decreasing amount of associated gas available.
As such, a need exists for a simple, energy efficient, and economic way of providing gas-based fluids for enhanced oil recovery.